P440: THE COMBINATION VENETOCLAX PLUS MCL1 OVERCOMES DRUG RESISTANCE IN ACUTE PROMYELOCYTIC LEUKEMIA
Authors: D. A. Pereira-Martins, N. I. Noguera, T. Ottone, A. Diepstra, C. Ortiz, I. Weinhauser, G. Huls, J. J. Schuringa, E. M. Rego, E. Ammatuna
Journal: HemaSphere (2022)

Background: Although acute promyelocytic leukemia (APL) is the most favorable subtype of acute myeloid leukemia, it remains a life-threating disease for high-risk patients. Despite progress in APL with arsenic (ATO) plus retinoic acid (RA) as front-line treatment, approximately 10-15% of patients will acquire resistance to ATO and/or RA and relapse. Relapse after ATO/RA based therapy remains very challenging, and novel drug schemes to overcome resistance are urgently needed. Analysis of bone marrow (BM) biopsies of 15 consecutive APL patients at diagnosis, revealed a homogeneous staining for BCL2 in all cases. These results suggest BCL2 inhibition could be a therapeutic option for resistant APL. In vitro treatment with the BCL2 inhibitor Venetoclax (VEN), resulted in increased MCL1 expression, suggesting a resistance mechanism to this therapy. Aims: Here, we investigated the effects of VEN+MCL1 inhibitor S64315 in APL cells resistant to ATO/RA, and evaluated the effect of the combination therapy in vivo, using a cell line derived xenograft (CLDX) model for APL with the ATO-resistant associated mutation PML A216V. Methods: Primary leukemic blasts from APL mice (n=4) and patients (age, 25-52y; n=10) were treated with VEN (250-500nM), ATO, ATRA (1µM), and S64315 (10-50nM) monotherapies and combinations and evaluated for cell survival. In addition, we evaluated the transcriptome of 16 APL patients (age, 25-74y; 7 males) enrolled in the TCGA study regarding the BCL2 expression. NB4/NB4R2 (RA-resistant) cell lines were transduced with PML A216V. Additionally, U937 cells were transduced with the inducible PML-RARA system. After induction of PML-RARA expression with Zinc for 0-24h, BCL2 protein levels were evaluated by western blot. Additionally, RA/ATO treated (1µM) NB4 cells for 6-72h were evaluated for BCL2/MCL1 levels. For the CLDX model, NB4 PML A216V cells were transplanted into NSG mice, and after chimerism detection (hCD45+) in the peripheral blood, mice were equally distributed into 4 groups of treatment: vehicle, VEN (25mg/kg/day), S64315 (20mg/Kg/day) and combination (n=5/group) for 21 consecutive days. Results: Gene set enrichment analysis using the transcriptome of APL patients with BCL2high associated these patients with the terms “EPPERT_HSC_LSC” and “17_LSC”. These results suggest that APL patients with high BCL2 expression differ in their metabolic and proliferative state compared to patients with BCL2low. Functionally, the induction of PML-RARA expression in U937 cells resulted in increased BCL2 expression in a time-dependent manner (peak at 6h). Contrarily, treatment with ATO/RA resulted in decreased BCL2 levels with upregulation of the MCL1 protein. BH3 profile revealed that ATO/RA treated NB4 cells displayed an increase in the MCL1 anti-apoptotic dependency. Additionally, in vitro and ex vivo treatment of APL cells (cell lines and primary blasts) revealed synergic activity of S64315, but not the BCL2 inhibitor (VEN), in combination with ATO. Furthermore, treatment of ATO resistant NB4 cells (by gradual exposure to ATO or by expression of the PML A216V) revealed sensitivity of those cells to the combination VEN+S64315. Finally, in vivo CLDX model revealed limited effect of VEN and S64315 monotherapy, while the combination was able to effectively reduce the leukemic burden, resulting in increased overall survival (vehicle:15 vs Combo:42 days). Summary/Conclusion: The combination of BCL2 and MCL1 inhibition is a potential effective treatment in ATO/RA resistant APL patients and could be clinically explored in this category of patients with very poor prognosis.